In order to comply with ever stricter emissions limits, it is necessary in automotive technology reliably to identify defects in a tank ventilation system of a motor vehicle. This prevents fuel vapors from escaping unnoticed from the tank ventilation system.
A method for detecting a leak in a tank ventilation system is known from patent specification U.S. Pat. No. 5,263,462. The method takes advantage of the natural formation of a vacuum inside the tank ventilation system after the vehicle has been switched off. According to the method, after the motor vehicle has been switched off, the reduction in the coolant temperature is monitored. If the coolant temperature falls below a specified value, a check is made as to whether a pressure switch arranged in the tank ventilation system has closed. Closure of the pressure switch indicates a reduction of the pressure inside the tank ventilation system below a predefined pressure value. When the pressure switch is closed, a leak inside the tank ventilation system can be ruled out since a natural vacuum has been able to form inside the tank ventilation system as a result of the cooling. An open pressure switch on the other hand is assessed as being an indication of a leak inside the tank ventilation system. In the case of a defective pressure switch, however, incorrect diagnoses can emerge. For example, in the event of the pressure switch clamping in the closed state, a leak in the tank ventilation system cannot be identified.